1. Field of the Invnetion
The development and testing of missiles with particular reference to improvements in means for providing targets which simulate the microwave and infrared energy emission characteristics of a source being tracked by a missile.
2. Discussion of the Prior Art
Devices have been known in the past which provide simulated targets for the testing of missiles in a laboratory environment where the emission of the target occurs in both the microwave and infrared energy portions of the spectrum. One method used in the past employs an incandescent lamp bulb mounted on the top or upper edge of a microwave (MW) horn and antenna of standard size. Although used with some degree of success, the major disadvantage of this method is that the microwave and infrared sources do not appear to originate from the same point; not being co-located, the energy being emitted gives the appearance of perhaps not one but at least two missiles somewhat spatially separated from each other.
Another method of providing dual spectrum simulated targets for laboratory use which has achieved satisfactory results, consists in placing a small microwave antenna, either a miniature horn or poly-rod antenna, in front of and in the middle of a collimating lens. In such a system, the infrared source is located behind the lens.
A third method used to simulate targets having emission characteristics in dual parts of the spectrum consists of placing two microwave horns, one above and the other below, a collimating lens with the infrared source located behind the lens. In systems adopting this third method, microwave energy of equal power and phase is fed to each horn with the result that the microwave target appears to originate from a point exactly between the two horns. With the apparent position of the microwave horns adjusted to appear as though occurring in coincidence with the axis of the infrared source, the appearance is given that both the microwave and infrared energy originate from the same point in space.
The second and third methods discussed hereinabove have the following limitations and disadvantages which are common to both methods. That is, they both suffer from an extremely narrow viewing angle and/or a very small viewing area. While suitable for simulating stationary targets, severely high accuracy pointing requirements are imposed upon the target motion device when the target must be moved over angles of .+-.25 degrees relative to the missile being tested. In the case of the collimating lens, these limitations and disadvantages are somewhat unattractive because the lenses are heavy and fragile in addition to being very expensive. Malfunctions of the target motion device have been found to be very costly if the lens is destroyed.
Also in connection with any infrared collimating lens, even when provided with an antireflection coating, the collimating lens has the characteristic of reflecting some of the infrared energy emitted by the missile being tested. In such cases, therefore, the missile, whose components are being subjected to the test to determine its reliability, sees in the collimating lens its own image by virtue of the reflected energy; the problem arises when the infrared shutter, that is, the shutter between the lens and the infrared source, is closed. It can easily be appreciated that during such a period, the missile in effect is still locked onto a target --but in effect it "sees" itself by virtue of the heat reflected by the lens. Such a condition is commonly referred to the "narcissistic effect."
The practice of placing a small microwave antenna, whether miniature horn or poly-rod, in front of and in the middle of a collimating lens, has the further disadvantage found almost impossible to overcome. Due to the fact that the microwave antenna must be of very small physical size in order to reduce to a minimum the obscuration of the infrared lens, such antennas are known to be very low in gain (6dB), in the case of horn antennas, or very limited in gain with band width (1-15%) in cases using poly-rod antennas.
Still other disadvantages have been found in methods in which the microwave target is made to appear from a point directly between two horns, one superior to and the other inferior to, a collimating lens cooperating with the infrared source located behind it.
It must be appreciated that the two microwave horns used in such an arrangement, produce grating lobes or beams that are not separated significantly from the position of the principal or main lobe. Such close proximity of the principal lobe to the grating lobes has been found to restrict the area or window in which the missile under test can be moved about without introducing the undesirable possibility of falsely locking onto and tracking onto the side lobes rather than the main lobe.
A further failing found to be present in the two microwave horns vertically disposed approach to dual spectrum targets is that the integrity of the co-locating of the infrared and microwave targets is not readily apparent. The absence of coincidence of the apparent infrared and microwave targets may occur, for example, if the power being supplied to either one of the two microwave horns changes. At such times the power fed to both horns is not balanced, and the apparent microwave position of the target will appear to move toward the horn which has the higher power output. Also, it has been found that if the phase of the power to either of the two horns undergoes a change, the microwave power available to the missile being tested is reduced. In either of these two cases, the system suffers from a loss of calibration which is not readily apparently by physically observing the apparatus. To reestablish or reconfirm system integrity, it is necessary for the user to submit the apparatus for extensive recalibration tests.